The microstructure sensitive multistage fatigue model captured the fatigue life of a powder metal FC-0205 steel alloy. Uniaxial strain controlled fatigue data and microstructure information from sets ...of high and low porosity specimens calibrated the model. Strain-life behaviour depicted that above the plastic strain limit of 0·002 mm mm
−1
in the low cycle fatigue regime, where ubiquitous plasticity occurred, the different porosity levels gave distinct, visibly different results. However, specimens tested below the plastic limit in the high cycle fatigue regime, where failure was dominated by local cyclic microplasticity, showed unclear fatigue lives at different porosity levels. Fractography using scanning electron microscopy showed no clear presence of striations; however, asserted striations in powder metal specimens were similar to geometrical features observed on fracture surfaces of monotonically loaded specimens. The experimental and microstructure data calibrated a fatigue model that allowed for satisfactory prediction of the varying porosity specimen strain-life curves.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
This paper provides the first description of residual stress distributions in a 66-mm-thick AA6061 deposit produced by additive friction stir deposition (AFSD). The AFSD process is being quickly ...developed as a solid-state additive manufacturing technique. Residual stresses in additively manufactured components can affect the susceptibility to fracture and fatigue, so there is a need to understand the residual stress distributions in the AFSD deposits. In this research, a large (194 × 49 × 66 mm) AA6061 deposit was produced using AFSD to study the residual stress distributions through its thickness. Three components of residual stresses were independently measured using neutron diffraction on the VULCAN instrument at Oak Ridge National Laboratory. At the starting end and the center region in the AA6061 deposit, the longitudinal residual stresses were generally tensile and ranged between − 5 and 91 MPa, and were consistently larger than the transverse residual stresses, ranging between − 5 and 76 MPa. The balancing compressive residual stresses were present at the finishing end of the deposit between − 127 and − 46 MPa. Residual stresses were homogeneously distributed throughout the deposit except at the top few layers (top 10 mm), which were exposed to the least number of thermal cycles from the AFSD process. The measured residual stresses in the AFSD AA6061 deposit were still significant in light of the low yield strength of AFSD AA6061 in its as-deposited state.
This work examines the grain refinement and mechanical properties of additive friction stir layer welding (AFSLW) of a Mg Alloy. The AFSLW process was used to additively fabricate a seven-layer tall ...build using 5-mm-thick plates of AZ31 Mg alloy, with each layer containing three parallel welding passes. This multipass AFSLW process demonstrates the ability of this additive method to produce tall and wide structures free of volumetric defects. While the AFSLW material was found to have lower monotonic tensile and fatigue strengths as compared to wrought Mg AZ31-H24, this result was expected due to the heat input from the additive layer welding process that relaxed the cold work strengthening mechanisms. Metallography cross sections and analysis via scanning electron microscopy revealed that the AFSLW build did not contain any detectable defects in any of the 21 weld passes. Furthermore, postmortem fractography found fatigue crack initiation mechanisms to be similar in both the AFSLW and wrought AZ31-H24. The results of this study indicate that large, defect-free builds with predictable mechanical properties can be achieved using the AFSLW process.
Microstructure-affected micromechanisms of fatigue crack growth operating near the limit plasticity regime were experimentally identified for the four main commercial high-pressure die-cast (HPDC) ...magnesium alloys: AM50, AM60, AZ91, and AE44. These fatigue micromechanisms manifested by the concomitant effects of casting pores, interdendritic Al-rich solid solution layer,
β
-phase particles, Mn-rich inclusions, rare earth-rich intermetallics, dendrite cell size, and surface segregation phenomena. These concomitant mechanisms clearly delineated the fatigue durability observed for the AM50, AM60, AZ91, and AE44 Mg alloys in both the low- and high-cycle fatigue regimes.
Electrical discharge machining (EDM) is advantageous in machining high strength and high-aspect-ratio components. However, EDM-induced thermal damage would detrimentally reduce fatigue life of ...critical components, such as biomedical Nitinol implants. As such, this paper focuses on the relationship between surface integrity and fatigue life of the wire-EDMed Nitinol samples. Surface integrity of Nitinol samples machined at main cut (MC) can be significantly improved by subsequent trim cuts. Nitinol samples at finish trim cut (TC) show lower surface roughness and thinner white layer than those at MC. The fatigue testing results show that the samples made by finish TC have 48% more fatigue life than those by MC. The examination of the fractography shows that fatigue cracks initiate from microvoids in the thick white layer. In addition, a thinner white layer or less tensile residual stress benefits fatigue performance, and would lead to longer fatigue life.
Experiments and modeling were performed to quantify the microstructure-fatigue properties relationship of wrought aluminum-lithium alloy 2099 (AA2099) plate. Microstructural morphology was examined ...via optical microscopy, SEM, and TEM techniques. The mechanical properties were determined through monotonic tensile and fully reversed fatigue testing. The rolled AA2099 was shown to have large elongated columnar grains several millimeters long. Metastable secondary phases, T1 and δ′, were observed and provided strength to the material. Fractography determined fatigue cracks initiate from copper-rich constituent particles. These particles ranged from 1 to 25 μm in size, with an average size of 12 μm. The size of these intermetallic Cu-rich particles influenced the fatigue life of AA2099. Lastly, a microstructure-sensitive fatigue model was used to correlate the various effects of intermetallic particle sizes on the fatigue life of AA2099.
•Strain-life experiments were conducted for AA2099.•The intermetallic particles served as crack nucleation sites.•Crack bifurcation, deflection, grain boundary delamination features were observed.•A multi-stage fatigue model captured the fatigue nucleation and small crack growth.
Expeditionary airfield matting systems are lightweight, portable surfaces that enable the rapid deployment of infrastructure to support aircraft operations. Individual matting components are ...assembled via interlocking joints to construct arrays that serve as temporary aircraft operating surfaces. The paper outlines the homogenization of the AM2 portable airfield matting system and its interlocking mechanisms to permit computationally efficient analyses toward understanding mechanisms that influence the global behavior of these arrays and underlying subgrade during aircraft maneuvers. An equivalent orthotropic two-dimensional continuum was developed from finite element analysis of a detailed three-dimensional model and its flexural behavior was validated against experimental data and solid finite element models. Interlocking joints were characterized using node-to-node connector elements based on subscale finite element studies. Both components were implemented into a full-scale model representative of a typical test section, and responses to static high tire pressure aircraft loads were analyzed over a soil foundation representing a California bearing ratio of 6%, yielding promising agreement with experimental data. Results of this study reveal an inherent coupling between load transfer, mat deflection, and near-surface subgrade stress with dependence on tire location, mat core shear flexibility, and joint stiffness.
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